1. Field of the Invention
The present invention relates to machining of parts, and more particularly to methods for machining parts shaped as bodies of revolution through multi-cutter turning.
This invention can be used in turning of non-rigid shafts having a complex longitudinal profile and requiring both high accuracy and surface quality.
The invention is also applicable to machining of shafts on program-controlled lathes in small-lot production of machine parts.
2. Description of the Prior Art
In turning of machine parts such as non-rigid (flexible) shafts in which the length-to-diameter ratio is greater than 5, the cutting forces give rise to an elastic flexure (deformation) in the cross section of the shaft being machined, which results in a less accurate longitudinal shape of the shaft.
The flexure of the shaft being machined can be minimized by resorting to multi-cutter turning in which machining is effected simultaneously by several cutters uniformly spaced along the circumference, the axis of which is in line with that of the shaft. This decreases the resultant of the cutting forces acting in the cross section and causing the elastic flexure of the shaft during machining.
There are known methods of multi-cutter turning with longitudinal stock removal distribution, wherein all cutters are forming cutters and all have the cutting points thereof set in a same plane perpendicular to the axis of a workpiece at a distance therefrom equal to the radius of a finished part. As meant here longitudinal stock removal distribution is that in terms of chip thickness.
In these multi-cutter turning methods, the height of micro-roughnesses characterizing the roughness (quality) of the surface of a part being machined is governed by the accuracy of radial and longitudinal positions of each cutter.
The error in the radial positioning of cutters must not exceed the specified height of micro-roughnesses of the surface, whereas the tolerance on the longitudinal positioning of cutting points is determined from the admissible height of micro-roughnesses, account taken on the entering angle of all the forming cutters.
However, even a highly accurate positioning of all the cutting points in the above directions is not a sufficient precondition for attaining specified surface roughness and machining accuracy.
In particular, the unequal angles of sharpening and the different blunting of the cutters, even with accurately positioned cutting points, give rise to un-balanced cutting forces in the cross section of the workpiece, which cause an elastic flexure of the workpiece between its supports on the lathe.
In addition, in turning by several forming cutters uniformly spaced along a circumference, the axis of which fails to coincide with, because of the runout of the locating surfaces of a workpiece, that of the workpiece by the magnitude of the eccentricity thereof, the depth of cut of each cutter varies during one revolution of the workpiece by the double magnitude of the eccentricity of the workpiece. This results in a transversal spring-back of the workpiece, with the effect that one or two cutters lose in succession their forming capability during one revolution of the workpiece, and the machined surface is lobbed crosswise.
Therefore, multi-cutter turning with longitudinal stock removal distribution produces no quality surface finish, whereas the accuracy of machining depends on the runout of the workpiece.
Also known are methods of multi-cutter turning with a stock removal depth distribution, wherein the cutting point of one of the cutters, which is a forming cutter, is set at a distance from the axis of a workpiece equal to the radius of a finished part.
The cutting points of the other--the non-forming--cutters are arranged at distances from the axis of the workpiece within the stock being removed, but different from the radius of the finished part, in a manner that the stock left for machining is distributed equally between the non-forming cutters.
With respect to longitudinal feed, the cutters are successively offset by a length which exceeds the longitudinal feed of the forming cutter per revolution of the workpiece divided by the number of the cutters and depends on the depth of cut and the entering angle of the cutters.
That in the above method the cutting point of only a single cutter is set at a distance equal to the radius of a finished part provides a quality surface finish. However, as the cutting points of the non-forming cutters are offset with respect to longitudinal feed through a distance, which, account taken of the entering angle of the cutters of 45.degree. or less, may exceed the depth of cut of the cutters, no machining of stepped and complex-shaped profiles is possible.
Also, in multi-cutter turning with stock removal distributed among cutters, that cutter which removes stock to a depth described by the contour of a finished part, presses against the part with a force whose magnitude varies as a function of the shape and the hardness of the part in the direction of longitudinal feed.
A variable force on one of the cutters disturbs the previous transversal equilibrium of forces and decreases the accuracy of machining through flexure of the part.
And so, methods involving stock removal depth distribution among cutters provide a quality finish, but fail to ensure adequate machining accuracy which depends heavily on the variations of the profile and of the hardness of a workpiece in the longitudinal section (in the direction of the longitudinal feed).
Forces acting upon a workpiece from cutters can be balanced, depending on the way stock removal is distributed among the cutters, by either of two cutter motion techniques: longitudinal stock removal is distributed by a longitudinal adjustment of all the cutters, whereas stock removal depth is distributed by adjusting the radial positions of the non-forming cutters.
For equal un-balanced forces, the longitudinal feed adjustments are much smaller, than the radial adjustments, so that the former can be completed with greater effectiveness and in a shorter period of time.
In known multi-cutter turning methods, the cutting forces or their components acting from the cutters on a workpiece are determined by measuring the forces or their components as reactions of cutter holders in tool carriers.
The above procedures for measuring the forces in multi-cutter turning of non-rigid parts are inaccurate, as what is measured is the forces acting upon the cutters, with the workpiece deformed elastically by the transversally unbalanced cutting forces.
Also, the measurement of the forces acting upon the cutters in the machining zone is affected by such variable factors as cutting temperature, cutter cooling temperature and others.
The measurement and the subsequent comparison of the forces in the cutter holders is also affected throughout the range of the measured cutting forces by the accuracy of load cells, amplifiers and control devices, this additionally impairing the accuracy of force balancing in multi-cutter turning.
The above methods require continuous control of the cutting forces using their measured values. The continuous measurement of the cutting forces during machining is also made difficult by entangled continuous chip and is ineffective in application to the turning of intermittent surfaces of parts.